The present invention relates to an arrangement of the components in a circuit, in order to prevent the electronic components in the circuit from being damaged by an over-voltage. The arrangement is especially suitable for high electric power devices requiring a high voltage processing operation or usually having to be installed outdoors so as to protect the component elements thereof.
Previously, the practice of preventing over-voltage damage was usually conducted using a photo coupler (such as photo-transistor coupler and photo-thyristor coupler etc.) as a coupling element between an input terminal and an output terminal of an electronic circuit for the purpose of isolating the input terminal and the output terminal. Generally, the breakdown voltage of the optimal photo coupler recognized in the art is approximately up to 5,000 VAC. Therefore, devices having such couplers are free of any problems as long as the over-voltage thereof does not exceed 5,000 VAC. However, once a high voltage over 5,000 VAC appears across the input and the output terminals of the circuit, the high voltage will go from the input (or output) side to the output (or input) side through the photo-thyristor and the electronic elements of the circuit. In such case, the photo-thyristor and the electronic elements will be damaged, and the entire printed circuit board may even be burned out. It is clear that over-voltage protection between the input terminal and the output terminal of a circuit is of vital importance. In view of the fact that the maximum breakdown voltage of existing photo-couplers is no more than 5,000 VAC, the present invention develops a specific circuit arrangement based on a series of experiments.